1. Field of the Invention
The present invention relates to an Nb solid electrolytic capacitor and a method for preparing the same.
2. Description of the Prior Art
Heretofore, a solid electrolytic capacitor has comprised an anode, a dielectric, an electrolyte layer, and a cathode. Generally, a solid electrolytic capacitor has had a structure comprising an anode made of a metal exhibiting valve action (valve metal), an oxidized layer as a dielectric layer formed over the surface of the anode, a solid electrolyte layer as a semiconductor layer formed on the dielectric layer, and a cathode made of graphite or the like formed on the solid electrolyte layer.
In this connection, the valve metal means a metal capable of forming an oxidized layer whose thickness can be controlled by anodic oxidation. Specifically, valve metal includes niobium (Nb), aluminum (Al), tantalum (Ta), titanium (Ti), hafnium (Hf) and zirconiun (Zr). Actually, however, aluminum and tantalum are mainly used.
With respect to Al, a foil is generally used as the anode, and with respect to Ta, a porous body prepared by sintering a Ta-based powder is used as the anode.
Of those solid electrolytic capacitors, a solid electrolytic capacitor of a porous sintered body type is particularly adaptable to miniaturization and capable of being adapted to have a high capacity, and hence there is strong demand therefor as a part which meets needs of miniaturization of a cellular phone, information terminal equipment or the like.
Recently, however, use of Ta solid electrolytic capacitors has not been expanded. This is because the annual output of Ta, which has been used for forming an anode and a dielectric of a Ta solid electrolytic capacitor, has been small owing to the fact that the amount of deposits of Ta is estimated to be small, and in consequence, the price of Ta has become increasing high. It is hardly expected that the price of Ta will decline hereafter. Accordingly, an alternative to Ta has been sought as a material for a solid electrolytic capacitor.
As the alternative, Nb which exhibits performance comparable to that of Ta when used in a solid electrolytic capacitor is considered to be a highly potential material. The amount of Nb deposits is estimated to be 100 times that of Ta deposits or more, and Nb is thus inexpensive as compared with Ta. Further, since Nb is distributed and outputted in various parts of the world, Nb is expected to be stably available.
Moreover, Nb has a specific gravity as low as about a half of that of Ta, and this enables further weight reduction of a solid electrolytic capacitor to be realized. Accordingly, it is expected that an Nb solid electrolytic capacitor will be mainstream from now on.
In the following, a typical preparation method of a conventional Nb solid electrolytic capacitor will be described with reference to FIG. 10.